More particularly, the electric motor concerned is of the type whose mechanical characteristic, i.e. the speed of rotation as a function of the resisting torque, is different in one direction of rotation than in the other.
The motor of the considered type comprises, in the known manner, a pair of brushes placed between two field poles and cooperable with an armature structure consisting of a commutator rigid with an armature support through which the shaft of the motor extends and in which are formed slots in which are wound connection wires of a series of strips or segments from which the commutator is formed, the armature support being rotatable about its axis between two permanent magnets.
It is known that in some window-raiser devices devoid of a balancing spring, or when the latter is eliminated for reasons of cheapness, the mechanism of the window raiser must overcome resisting forces which are higher when rising (friction forces to which is added the weight of the window glass) than when descending (friction forces from which the weight of the window glass is subtracted since the glass becomes driving means). Further, the shock-absorbing capacity at the end of the travel of this mechanism is higher when reaching the upper stop. It will be understood that excessive descending speed unnecessarily increases the fatigue of the mechanism which must be designed to take this into account.
Moreover, in the case where the purpose is to control or drive an element formed by a seat back, it is desirable to arrange that the blocked torque, i.e. the torque exerted by the motor when the lower stop is reached and the seat back has reached its lowermost position, be not too high so as to ensure conditions of safety for the passenger located behind the lowered seat back.